1. Technical Field
This invention relates generally to a pneumatic tire and in particular to a tire for an automobile, light truck or the like and having a specific tread configuration.
2. Description of the Prior Art
A properly mounted tire is subject to vehicle operating conditions in which inner and outer halves of the tire are subject to different and diverse loads. For example, during a cornering maneuver of the vehicle, a lateral acceleration of the vehicle transfers more load to each tire located to the outside of the vehicle during a turn. During such a maneuver the wheels of the vehicle on which the tires are mounted may not remain perpendicular to the road surface on which the vehicle operates. This tendency for the wheels to not remain perpendicular to the road surface results in an angle between a line extending normal to the road surface and the plane of a wheel which is defined in the industry as the camber angle. Furthermore, the outer half of each tire located to the outside of the vehicle during a turn is subjected to a larger normal force than the inner half of the tire. The tire deforms laterally with respect to the wheel plane adjacent to tire-road interface during a turn. The combination of lateral deflection of the tire, the camber angle of the wheel and a normal force distribution, for example, produces the different operating conditions between the inner and outer halves of the tire.
The different operating conditions for the inner half of the tire compared with the outer half of the tire have been compensated for by asymmetrical tread patterns. For example, U.S. Pat. No. 3,155,135 discloses a tire tread having a rib pattern on one half and a block pattern on the other half of the tread. These different tread pattern halves compensate the tire for differences in vehicle operating conditions by providing improved contact with the road surface.
U.S. Pat. No. 4,848,429 discloses a tire with a tread portion having circumferential ribs on one half of the tread and a tread block pattern on the other half of the tread. In U.S. Pat. No. 4,848,429 the block pattern of the tread is for maximizing traction and other road-holding performance features during adverse environmental conditions such as snow covered or frozen ground. The ribs of the tread assure good tire performance under clear, dry or wet road conditions. An overall value of the ratio of tread contact area to the gross area in the contact patch between the tire and the road (contact surface ratio), for nominal operating conditions of load and inflation pressure is between 0.50 and 0.80. However, the contact surface ratio for the outside tread portion has a relatively high value, compared with the average value of the tire, to increase its gross area in contact with the ground surface. The results of U.S. Pat. No. 4,848,429 were lessened through a "greater disposition for having a more rapid and irregular wearing-out of the tread". Other aspects of the tire disclosed in U.S. Pat. No. 4,848,429 compensate for different operating conditions for the inside tread portion and the outside tread portion of the tire by including diverse elastomeric compositions and diverse tread thicknesses of the two tread portions.
U.S. Pat. No. 4,785,863 discloses differening tread patterns on either half of the tire tread. One pattern is particularly suitable for use on a dry road and the other pattern is suitable for use on a wet road. The rigidity of the land portions partitioned by lateral inclined grooves is different between both sides of the tire tread. The half with a relatively large pitch length of lateral grooves and a small inclination angle enables relatively good traction on dry roads due to wide land areas of high rigidity. The half with a relatively small pitch length of lateral grooves and large inclination angles enables relatively good traction on wet roads as a result of narrow land areas of low rigidity. Improved steering controllability and stability on both wet and dry roads were obtained with large pitch lateral inclined grooves and small inclination angles being to the outside of the vehicle. For this outside tread pattern portion the ratio of the contact area to the groove in the tread contact patch is relatively large and the rigidity of the land portion is high.
The tread configuration disclosed in U.S. Pat. No. 4,840,210 reduces abnormal shoulder wear by having the diameter of the tread outer shoulder larger than the diameter of the tread inner shoulder. The shearing rigidity of the tread on the tire outer shoulder is larger than the shearing rigidity on the inner shoulder by varying the groove depth across the tire to offset an undesirable lateral force induced by the different shoulder diameters. The result of this design appears to add an unsymmetrical mass to the tire and a built in conicity.
Japanese Publication No. 63-159108 discloses a tread surface having a plurality of circumferentially extending ribs where the contact surface ratio in an axial direction varies. This tread pattern configuration enhances the traction characteristics of the tire on dry road surfaces and makes abrasion of the tire uniform. Further, the tire is mounted "inside out" for running on a wet road surface to obtain similar performance characteristics to that of a symmetrical tire tread pattern running on a dry road surface. This "inside out" running requires cumbersome and time consuming dismounting and remounting of the tires on a rim. The tread pattern has no symmetry of rib surface contact areas about a midcircumferential plane of the tire.
What is generally desired for good performance from a tire, whether operating in straightaway driving, cornering, accelerating or braking condition, is a strong directional stability or handling, very low rolling resistance, excellent riding comfort, relatively low noise emission, relatively high milage tread life and relatively good traction. Also, the mass of the tire should be as low as possible for the loading range specified without decreasing the endurance or durability of the tire. Vehicle operating conditions such as cornering and camber conditions vary forces widthwise across the tire contact area and should be compensated for in a tire along with the vehicle operating conditions of straightaway driving. Furthermore, vehicle suspension systems are such that unsymmetrical as well as symmetrical deflections and angles are imposed on the vehicle tire-wheel system. Symmetrical deflections and angles imposed on a asymmetrical tire tread is a performance challenge not solved by the prior art.
Therefore, there is a need for a new tire with an asymmetrical tread having optimum tire performance during asymmetrical vehicle operating conditions as well as optimum performance during symmetrical vehicle operating conditions such as straightaway driving. The asymmetrical tire tread being more desirable than the symmetrical tire tread due to the criticality of asymmetrical vehicle operating conditions on the life of the tire. A mud and snow rating is also desirable for the tire, especially in light truck operations.